This invention relates to tunable resonant structures and especially to an optically tunable resonant structure which can support electromagnetic (EM) oscillations within a frequency range of about 10 GHz to 1000 GHz.
Conventionally tunable resonant devices are used in many applications, such as directional filters, channel-dropping filters, directional couplers, and traveling-wave modulators. Such resonant devices are mechanically or electrically tunable. For example, mechanical tuning includes the insertion of a flat dielectric material into a ring resonator, and a series of slits across a circular resonator. Electrical tuning features the application of an electrical control signal to a resonant structure.
Conventional electrically tunable resonators use ferrite, diodes or PIN semiconductor devices as an interaction material to induce a change in the frequency of EM oscillations.
The operation of the ferrite resonator is dependent upon the interaction between a slab of ferrite material and a magnetic biasing field for its frequency-changing effect. Ferrite materials cause a relatively high attenuation of EM energy at millimeter wavelengths.
The diode resonators employ one or more diodes mounted inside a resonant structure. The diodes are responsive to a D.C. bias voltage applied across the diode electrodes. The field produced by the bias voltage induces a change in the electrical characteristics of the diode which, in turn, affects the impedance at various points within the resonant structure. The change in impedance causes a change in the resonant frequency of the resonator. At frequencies above about 60 GHz, the internal dimensions of the resonator are relatively small so that accurate positioning of a diode is a problem. Also, the attenuation of EM oscillations by a variable reactance diode increases with increasing frequency above approximately 60 GHz.
A PIN semiconductor resonator is a slab of variable-conductivity semiconductor material in contact with a portion of the surface area of one of the walls of the resonator. The microwave conductivity of the semiconductive slab is responsive to the polarity of a D.C. bias voltage applied across the slab electrodes. The polarity of the applied bias voltage changes the conductivity of the slab and causes the resonant properties of the slab to change.
These conventional resonant structures require the application of an electrical signal by either inductive coupling, such as by coils, to the ferrite, or by wiring to the diode or PIN semiconductor. Such applications require structures and circuitry, some of which must be attached to the interaction material and which may cause spurious interference and insertion loss to the frequency-changing performance. The circuitry typically includes isolation networks to prevent such interference. The structures and circuitry are costly and may be inconvenient for specific applications where space is limited.
The response time, that is, the time for the EM oscillation to shift in frequency in response to the electrical signal applied to the resonator, is slow for conventional resonators because the response time is dependent on the medium which conducts the electrical signal. The response time for the PIN semiconductor resonators is further dependent on the traversal of electron-hole pairs across its entire intrinsic region.